Line a large rimmed baking sheet with parchment paper. Stir cornstarch and 1 1/2 tablespoons cold water in a small bowl to blend. Combine fresh cherries and next 4 ingredients in a large saucepan. Cook over medium heat, stirring occasionally, until cherry juices are released, about 5 minutes. Add cornstarch mixture; bring to a boil, stirring often. Remove from heat and let cool to room temperature, stirring occasionally. 

Roll out pastry on a lightly floured surface to an 18×15″ rectangle. Using a sharp knife or pizza cutter, cut dough into nine 6×5″ rectangles. Whisk egg white and 1 tablespoon water in another small bowl for egg wash. 

Working with 1 pastry rectangle at a time, place on a work surface and brush edges with egg wash. Scoop 3 tablespoons cherry mixture onto one side; fold dough over filling so that short ends meet, forming a 5×3″ packet. Crimp edges with a fork to seal. Using a sharp knife, cut a few slits in top of pie to vent. Place on prepared baking sheet; repeat with remaining dough and filling. 

Brush tops with egg wash, then sprinkle with raw sugar. Chill for 30 minutes. Preheat oven to 375°. Bake pastries until tops and bottoms are golden brown, 30-40 minutes. Let cool for 10 minutes on baking sheet. Transfer to wire racks; let cool completely.